Hi,
 
we want to subscribe for presence information on a SIP trunk but for every SUBSCRIBE request we receive a 403 Forbidden Response.
 
I even tried using a SIP trunk between 2 CUCM clusters and configuring a BLF button on one trunk for a phone on the other cluster. Using Wireshark I sniffer the communication and see the same issue. So even between the 2 clusters I see the same SUBSCRIBE Request and 403 Forbidden Response.
 
I double checked the config on the SIP trunk and the Subscribe CSS is correct and trunk and phones are in the same presence group.
 
I also configured the same CSS on the trunk for incomming calls and I can call the phone on the other cluster over the SIP trunk so CSS and partition are definitely OK.
 
What other reason can give the 403 Forbidden?
 
CUCM version is 8.6
 
Regards,
Roland